U.S. patent number 3,922,789 [Application Number 05/531,707] was granted by the patent office on 1975-12-02 for boom length sensing system with two-block condition sensing.
This patent grant is currently assigned to Koehring Company. Invention is credited to Ivan D. Sarrell.
United States Patent |
3,922,789 |
Sarrell |
December 2, 1975 |
Boom length sensing system with two-block condition sensing
Abstract
The disclosure relates to a system for sensing the length of an
extendible boom of a crane and for sensing the impending occurrence
of a two-block condition through the use of a single electrically
conductive cable mounted on the boom of the crane. The cable is
wound on a spring-biased reel mounted for rotation on a
non-extending portion of the crane boom with one end of the cable
secured to and electrically connected to the uppermost movable
section of the crane boom. A constant current is supplied to the
cable through an electrical contactor arranged to permit movement
of the cable as the boom is varied in length. The constant current
is also supplied to a circuit in parallel with the cable and the
circuit senses a resistance value for the length of the cable
between the contactor and the end secured to the movable section of
the crane boom in response to the current flowing through the
cable. In the preferred embodiment of the invention, the cable is
connected to the upper-most movable section of the crane boom
through a normally closed switch. An arm pivotally mounted to the
peak of the crane boom cooperates with the switch and opens the
switch in response to an impending two-block condition. The circuit
in parallel with the cable senses an open condition of the normally
closed switch, thus providing a manifestation of an impending
two-block condition. To insure accurate sensing of boom length, the
contactor and cable are preferably fabricated from different
electrically conductive metals which produce a predetermined
thermal EMF characteristic at the contact point therebetween. The
thermal EMF characteristic is selected such that the thermal EMF
tends to cancel thermal effects from the resistance of the cable
due to temperature variations. A system for calculating boom load
limit values in response to boom length and boom angle values is
also disclosed.
Inventors: |
Sarrell; Ivan D. (Rising Fawn,
GA) |
Assignee: |
Koehring Company (Milwaukee,
WI)
|
Family
ID: |
24118715 |
Appl.
No.: |
05/531,707 |
Filed: |
December 11, 1974 |
Current U.S.
Class: |
33/702; 33/626;
116/202; 340/685; 212/278; 212/281; 33/655; 182/18 |
Current CPC
Class: |
B66C
23/905 (20130101) |
Current International
Class: |
B66C
23/90 (20060101); B66C 23/00 (20060101); G08B
021/00 () |
Field of
Search: |
;33/125R,185V,138
;340/267C ;212/39A ;116/124F |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Martin, Jr.; William D.
Attorney, Agent or Firm: Beck; Andrew J.
Claims
I claim:
1. Apparatus for sensing the length of an extendible boom of a
crane comprising:
a reel mounted for rotation on a nonextending portion of the crane
boom, said reel being electrically insulated from said boom;
an electrically conductive cable having a substantially uniform
resistance per unit length thereof, one end of said cable being
secured to and electrically connected to the uppermost movable
section of the crane boom and the other end of said cable being
wound on said reel;
biasing means for biasing said reel in a direction of rotation
tending to wind said cable onto said reel;
means mounted on said nonextending portion of the crane boom
adjacent said cable for electrically contacting said cable and
providing a moving electrical contact with said cable;
means for applying current to said contacting means to thereby
apply said current through a length of said cable between said
contacting means and said one end of said cable; and,
circuit means for sensing a resistance value for said length of
said cable between said contacting means and said one end of said
cable in response to said current flowing therethrough, and for
providing a manifestation of the length of the crane boom as a
function of the sensed resistance of said length of said cable
between said contacting means and said one end of said cable.
2. The apparatus of claim 1 wherein said cable and said contacting
means are fabricated from different electrically conductive metals
producing a predetermined thermal EMF characteristic at the contact
point therebetween, the thermal EMF characteristic being selected
such that the thermal EMF tends to cancel thermal effects on the
resistance of the cable due to temperature variations.
3. The apparatus of claim 2 wherein said cable is type 304
stainless steel and said contacting means is brass.
4. The apparatus of claim 2 wherein said electrical current is a
constant direct current supplied from a constant current
source.
5. The apparatus of claim 4 wherein said circuit means includes a
meter electrically connected between said contacting means and
ground to sense the voltage therebetween.
6. The apparatus of claim 1 wherein said electrical current is a
constant direct current supplied from a constant current
source.
7. The apparatus of claim 6 wherein said circuit means includes a
meter electrically connected between said contacting means and
ground to sense the voltage therebetween.
8. The apparatus of claim 1 including a normally closed switch
mounted on said uppermost movable section of the crane boom and
wherein said cable is electrically connected to said section
through said switch.
9. The apparatus of claim 8 including means mounted on said
uppermost movable section of the crane boom in cooperable relation
to said switch for opening said switch in response to an impending
two-block condition and wherein said circuit means includes means
electrically connected to said contacting means for sensing an open
condition of said switch.
10. The apparatus of claim 9 wherein said electrical current is a
constant direct current supplied from a constant current
source.
11. The apparatus of claim 10 where said circuit means includes a
meter electrically connected between said contacting means and
ground to sense the voltage therebetween.
12. The apparatus of claim 11 wherein said cable and said
contacting means are fabricated from different electrically
conductive metals producing a predetermined thermal EMF
characteristic at the contact point therebetween, the thermal EMF
characteristic being selected such that the thermal EMF tends to
cancel thermal effects on the resistance of the cable due to
temperature variations.
13. Apparatus for sensing the length of an extendible boom of a
crane and the impending occurrence of a two-block condition
comprising:
a reel mounted for rotation on a nonextending portion of the crane
boom;
an electrically conductive cable having a substantially uniform
resistance per unit length thereof, one end of said cable being
electrically connected through a normally closed switch to the
uppermost movable section of the crane boom and the other end of
said cable being wound on said reel;
means cooperable with said switch and mounted on said uppermost
movable section of the crane boom for opening said switch in
response to an impending two-block condition;
biasing means for biasing said reel in a direction of rotation
tending to wind said cable onto said reel;
means mounted on said nonextending portion of the crane boom
adjacent said cable for electrically contacting said cable and
providing a moving electrical contact with said cable;
means for applying an electrical current to said contacting means
to thereby apply said current through a length of said cable
between said contacting means and said one end of said cable;
circuit means for sensing a resistance value for said length of
said cable between said contacting means and said one end of said
cable in response to said current, and for providing a
manifestation of the length of the crane boom as a function of the
resistance of said length of said cable between said contacting
means and said one end of said cable; and,
circuit means for sensing an open condition of said normally closed
switch in response to said current flowing through said cable.
14. The apparatus of claim 13 wherin said electrical current is a
constant direct current supplied from a constant current
source.
15. The apparatus of claim 14 wherein said circuit means includes a
meter electrically connected between said contacting means and
ground to sense the voltage therebetween.
16. The apparatus of claim 13 wherein said cable and said
contacting means are fabricated from different electrically
conductive metals producing a predetermined thermal EMF
characteristic at the contact point therebetween, the thermal EMF
characteristic being selected such that the thermal EMF tends to
cancel thermal effects on the resistance of the cable due to
temperature variations.
17. The apparatus of claim 15 wherein said cable and said
contacting means are fabricated from different electrically
conductive metals producing a predetermined thermal EMF
characteristic at the contact point therebetween, the thermal EMF
characteristic being selected such that the thermal EMF tends to
cancel thermal effects on the resistance of the cable due to
temperature variations.
Description
BACKGROUND OF THE INVENTION
The present invention relates to cranes having extendible booms
and, more particularly, to a system for electrically sensing the
length of an extendible boom of a crane and for sensing the
impending occurrence of a two-block condition.
In operating a crane with an extendible boom, many factors are
important in determining the stability of the crane and the maximum
safe load conditions under various conditions of boom position. For
example, safe load conditions for the boom depend upon certain
dynamic operating conditions including boom length and boom angle.
The stability of the crane is also dependent upon these conditions
as well as other factors such as the orientation of the boom
relative to the body of the crane.
While the operator may be able to estimate factors such as boom
length and angle, more accurate indications may be required when
operating the crane near the maximum load and/or stability limits.
Accordingly, various systems have been devised to provide more
accurate indications of boom length, boom angle and boom
orientation. Such systems are typically complex and accordingly
quite expensive and may still provide less than optimum accuracy.
Moreover, known sensing systems typically provide single function
outputs, further complicating the overall indicating and/or control
system.
OBJECTS AND SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide a
novel boom length sensing system which is relatively inexpensive,
yet extremely accurate and reliable.
It is another object of the present invention to provide a novel
boom length indicating system which incorporates a two-block
condition sensing system therein.
It is yet another object of the present invention to provide a
novel boom length indicating system which provides a boom length
related output signal having characteristics readily usable for
both boom length indications and other more complex functions.
These and other objects and advantages of the present invention are
accomplished through the provision of a system which senses the
length of an extendible boom of a crane by sensing the changes in
resistance of a cable variable in length in response to variations
in boom length. One end of the cable is wound on a take-up reel
mounted on a non-extending portion of the crane boom and the other
end of the cable is secured to and electrically connected to the
upper-most movable section of the crane boom. An electrical
contactor mounted on the non-extending portion of the crane boom
electrically contacts the cable and provides a moving electrical
contact therewith. Current is applied to the contactor to supply
through a length of cable between the contactor and the end of the
cable connected to the upper-most movable section of the crane
boom. A circuit connected to the contactor senses a resistance
value for the length of the cable between the contactor and the end
of the cable connected to the upper-most movable section of the
crane boom in response to the current applied to the contactor. A
manifestation of crane boom length is provided as a function of the
sensed resistance.
In addition, the cable provides a means for sensing the impending
occurrence of a two-block condition through the provision of a
normally closed switch between the end of the cable and its
electrical connection to the upper-most movable section of the
boom. An arm positioned to be engaged by the load connecting block
at a point of impending occurrence of a two-block condition
cooperates with the switch and opens the switch in response to the
impending two-block condition. The open condition of the switch is
sensed in response to the current flowing through the cable and the
two-block condition can thus be avoided.
In accordance with the preferred embodiment of the invention, the
cable and the contactor are fabricated from different electrically
conductive metals thereby producing a predetermined thermal EMF
characteristic at the contact point therebetween. The thermal EMF
characteristic is selected such that the thermal EMF tends to
cancel thermal effects on the resistance of the cable due to
temperature variations. Such a characteristic is obtained, for
example, by employing a brass contactor and a type 304 stainless
steel cable.
The manner in which the present invention accomplishes these and
other objects and provides further advantages will become apparent
to one skilled in the art to which the invention pertains from the
following detailed description when read in conjunction with the
appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view in elevation of a crane equipped with a boom
length and two-block sensing system in accordance with the present
invention;
FIG. 2A is a view in elevation of a portion of the boom
illustrating the reel and contactor assemblies of the system of
FIG. 1 in greater detail;
FIG. 2B is a view in cross-section taken along the lines IIB--IIB
of FIG. 2A;
FIG. 3 is a functional diagram schematically illustrating the
system of FIG. 1 in greater detail;
FIG. 4 is a circuit diagram illustrating the constant current
source and boom length and anti-two-block circuit of FIG. 3 in
greater detail;
FIG. 5 is a circuit diagram illustrating another embodiment of the
boom length and anti-two-block circuit of FIG. 3; and,
FIG. 6 is a circuit diagram illustrating a load limit calculating
system in accordance with the present invention.
DETAILED DESCRIPTION
FIG. 1 illustrates a crane with an extendible boom employing the
system of the present invention to sense boom length and to sense
an impending two-block condition. Referring now to FIG. 1, a
take-up reel assembly 10 and a contactor assembly 11 are mounted on
the nonextending base 12 of the crane boom in a suitable
conventional manner. A sensing cable or wire 14 wound on the reel
assembly 10 extends outwardly from the reel and contactor
assemblies along the boom to the uppermost movable section or peak
16 of the boom and is there fixedly secured to the peak 16 by
suitable electrical insulator 18. The end of the wire 14 secured to
the boom peak 16 is electrically grounded to the boom peak through
the normally closed contacts of a suitable switch generally
indicated at 20.
An arm 22 or other suitable member is pivotally connected to the
boom peak 16 and is biased out of contact with the actuator of the
switch 20 as will hereinafter be described in greater detail. The
arm 22 extends beneath the underside of the boom peak and is
positioned to be engaged by a loading connecting means such as the
illustrated block 23 when the block is raised and approaches a
position of contact with the underside of the boom peak. The arm 22
is thereby moved into contact with the actuator of the switch 20,
opening the switch 20 and actuating anti-two-block circuitry
hereinafter described.
A cable 24 or other suitable means may be utilized to connect the
contactor assembly 11 to control and indicating circuitry located
in the cab 26 of the crane. Suitable insulators 28 may be connected
to various sections of the crane boom to support the wire 14 and
guide the wire as it is wound and unwound from the reel assembly 10
through the contactor assembly 11. Additionally, grounding straps
or wires may be provided between the boom sections to insure good
electrical contact therebetween. Typically, however, such
precautions are not necessary since there is usually an adequate
return path over the entire length of the boom.
A portion of the boom of the crane illustrated in FIG. 1 is
illustrated in greater detail in FIGS. 2A and 2B in order to
facilitate an understanding of the invention. Referring now to
FIGS. 2A and 2B, the reel and contactor assemblies 10 and 11
include a reel 30 mounted on a bracket 32 secured to the side of
the boom base 12. The wire 14 is wound on the reel and extends
through an insulated support member 34 and apertures 36 through the
assembly housing 38 to the connection 18 on the boom peak 16. The
reel 30 is conventionally journaled for rotation on the bracket 32
and is biased by a spring 40 or other suitable means so that the
reel 30 applies tension to the wire 14 and tends to wind the wire
14 onto the reel 30 as slack is available.
The reel 30 is preferably electrically insulated from the boom base
12 in a suitable conventional manner. For example, the bracket 32
on which the reel 30 is mounted may be insulated from the boom base
12 by a sheet of non-conductive or insulative material as is
generally indicated at 33. Insulative washers 35 and suitable
inserts may be provided to prevent contact between the bracket 32
and the fasteners used to mount the bracket 32 to the boom base
12.
The contactor assembly 11 includes a pair of rollers 42 each
journaled for rotation on an arm 44 connected to the housing 38 of
the assembly. The rollers 42 are mounted so that the wire 14 passes
therebetween in electrical contact therewith. To insure good
contact, the arms 44 may be spring members mounted so as to force
the rollers 42 toward one another and into firm contact with the
wire 14 running therebetween. In addition, each of the rollers 42
may be slightly fluted to receive and guide the wire
therebetween.
The arms 44 are insulated from the housing 38 in a suitable manner,
e.g. through the provision of an insulative arrangement 45 such as
that previously described in connection with the mounting of the
reel 30 to the boom base 12. Such an arrangement prevents
electrical contact between the rollers 42 and the housing 38 so
that the rollers are not grounded to the crane.
An electrical connection to the rollers 42 may be made through the
connection of an electrical lead or wire 46 to one or both of the
arms 44 supporting the rollers 42. To prevent electrical contact
between the wire 14 and the housing 38 where the wire 14 passes
through the apertures 36, suitable insulative grommets 48 or other
insulators may be mounted in the apertures 36 as illustrated. The
wire 46 may provide an electrical connection between the portion of
the wire 14 in contact with the rollers 42 of the contactor
assembly 11 and the circuitry in the cab of the crane thus
functioning as the transmission line 24 as was previously
described.
In the illustrated embodiment of the contactor assembly 11, the
rollers 42 are brass and are journaled for rotation in brass
bushings 47. The brass bushings are carried by the arms 44 which
are made of a sheet of brass, bent to provide a central portion 49
to which the electrical lead 46 is connected.
The wire 14 is preferably of a metal cable such as a commercially
available type 304 stainless steel seven strand aircraft control
cable which, at the point of contact with the rollers 42, generates
a thermal EMF which varies with temperature in a predetermined
manner and tends to cancel the affect of temperature variations on
the resistance of the wire 14. In other words, the point of contact
between the roller 42 (of one metal) and the wire 14 (of another
metal) provides thermo-couple action with the resultant generation
of a temperature responsive thermal EMF. By selecting the two
metals of the roller 42 and wire 14 such that the thermal EMF
varies with temperature in the same manner as the resistance of the
wire 14, errors in the length measurement due to changes in the
resistance of the wire 14 with temperature may be minimized. With
the brass roller and stainless steel cable described above, less
than a 3% total error has been obtained between temperatures in the
freezing and boiling ranges.
To facilitate an understanding of the operation of the present
invention, reference may be had to the boom length and two-block
sensing system according to the present invention illustrated
schematically in FIG. 3. Referring now to FIG. 3, the circuitry in
the cab of the crane includes a constant current source 52 and a
boom length and anti-two-block circuit 54 described hereinafter in
greater detail. The constant current source 52 may be any suitable
circuit for regulating current from an unregulated source such as
the vehicle battery indicated at 50.
A desired value of constant current, e.g. a constant 25 milliamp
current, is supplied from the constant current source 52 through
the boom length and anti-two-block circuit 54 and the contactor
assembly 11. The boom length and anti-two-block circuit 54 and the
portion of the wire 14 between the contactor assembly 11 and the
boom peak 16 are connected in parallel between the constant current
source output and ground, thereby presenting two parallel paths for
the flow of current supplied by the constant current source 52.
Accordingly, the current supplied from the constant current source
52 is divided between the two paths as a function of the impedance
presented by the two paths.
As the boom is extended and retracted, the length of the wire 14
between the contactor 11 and the boom peak 16 increases and
decreases. The increase and decrease in the length of the wire 14
results in an increase and decrease, respectively, in the
resistance of the wire 14 between the contactor assembly 11 and the
boom peak 16. Current from the constant current source 52 is thus
fed to two paths, one of which presents a constant impedance (the
circuit 54) and the other of which presents an impedance varying
directly with variations in boom length.
In addition, the normally closed switch 20 is opened by the arm 22
when the block 23 is raised to a position of engagement with the
arm 22, i.e., a position adjacent the underside of the boom peak
16, signaling an impending two-block condition. When the switch 20
is opened in this fashion, all of the current normally flowing
through the wire 14 is diverted through the boom length and
anti-two-block circuit 54 and is sensed as an impending two-block
condition. The boom length and anti-two-block circuit 54 may
disengage the power to the winch driving the block 23 (not shown)
or may otherwise indicate or prevent the impending two-block
condition.
One embodiment of the constant current source 52 and the boom
length and anti-two-block circuit 54 is illustrated in greater
detail in a schematic diagram of FIG. 4. Referring now to FIG. 4,
the constant current source 52 may be any suitable conventional
circuit for supplying a constant output current from the vehicle
battery. The positive output terminal of the vehicle battery 50 may
be connected through a resistor 56, a diode 58 and a resistor 60,
the base electrode of a PNP transistor 62 and to the emitter
electrode PNP transistor 64. The collector electrode of the
transistor 62 may be connected to the base electrode of the
transistor 64 and the collector-base junction may be grounded
through series connected resistor 66 and potentiometer 68. The
junction of the cathode electrode of the diode 58 and the resistor
60 may be connected through a diode 70 to the emitter electrode of
the transistor 62 and through a reverse poled Zener diode 72 to
ground.
The collector electrode of the transistor 64 supplies a constant
current to the boom length and anti-two-block circuit 54 and the
potentiometer 68 provides a selectively variable dropping
resistance as will hereinafter be described in greater detail. The
collector electrode of the transistor 64 may be connected directly
to the contactor assembly 11 to supply the constant current to the
wire 14. A fuse 74 or other over-current protection device may be
provided to protect the constant current source 52 from damage.
The coil of a two-block sensing relay 76 may be connected between
the collector electrode of the transistor 64 and ground to provide
for anti-two-block sensing. The collector electrode of the
transistor 64 may also be connected to one side of a normally
closed set of relay contacts 78 and to one side of a set normally
open contacts 80, the contacts 78 and 80 being associated with the
relay coil 76. The other side of the relay contacts 78 may be
connected through a suitable meter 82 and a rheostat 84 to the arm
of the potentiometer 68 and the constant current source 52. The
other side of the set of relay contacts 80 may be connected through
a suitable audible alarm and visual indicator 86 to ground and
through a relay coil 88 to ground.
In operation, the constant current source 52 supplies a constant
current output both to the contactor assembly 11 and to the boom
length and anti-two-block circuit 54. The total current supplied
from the constant current source 52 is divided between the two
circuit paths including the path through the contactor assembly 11
and the wire 14 and the path through the boom length and
anti-two-block circuit 54 as a function of the impedance of the
circuit. The impedance of the boom length and anti-two-block
circuit 54 is a constant once the potentiometer 68 and the rheostat
84 have been adjusted to indicate current changes directly on the
meter 82 as length changes Therefore, current flows through the
respective circuits remains unchanged unless the length of the wire
14 changes.
As the length of the wire 14 varies with variation in boom length,
more or less current flows through the movement of the meter 82
thereby providing an indication related to changes in the length of
the wire 14 and thus in the length of the boom of the crane. As was
previously mentioned, the meter 82 may be callibrated directly in
terms of boom length through adjustment of the rheostat 84 and
potentiometer 68.
A slight amount of current flows through the relay coil 76 but the
amount is insufficient to energize the relay. However, if the
switch 20 is opened through the sensing of an impending two-block
condition, a much larger proportion of the current from the
constant current source 52 is diverted through the relay coil 76
thereby opening the contacts 78 and closing the contacts 80. With
the contacts 80 closed, an audible and/or visual alarm may be
provided by the indicator 86 and the relay coil 88 is energized.
The relay coil 88 may operate contacts in the crane winch control
circuitry so as to deenergize the winch when the impending
two-block condition is sensed. Damage to the hoist cable and/or
winch as a result of a two-block condition may thereby be
prevented.
It should be additionally noted that the length related signal
supplied to the meter 82 may be utilized to provide additional
output signals such as boom extension velocity and acceleration.
For example, by differentiating the boom length signal with respect
to time, the boom extension velocity may be determined. Boom
extension acceleration may be determined by a similar conventional
technique. The length, velocity and acceleration signals may be
readily utilized in installations requiring dynamic analysis and
control functions such as that described hereinafter in connection
with FIG. 6.
Another form of the boom length and anti-two-block circuit 54 is
schematically illustrated in FIG. 5. Referring now to FIG. 5, the
constant current output signal from the constant current source 52
may be supplied directly to the contactor assembly 11 through a
test/run switch 90. The output from the constant current source 52
may also be connected through the two-block sensing relay coil 76
to ground and through a boom length indicating circuit 92 to the
arm of the potentiometer 68 (FIG. 4) and the constant current
source 52.
The boom length indicating circuit 92 may include a rheostat 94, a
meter 96 and a rheostat 98 connected in series between the output
of the constant current source 52 and the arm of the potentiometer
68. A set of relay contacts 100 and a current limiting resistor 102
may be connected in series across the meter 96 so as to bypass the
meter 96 when the relay contacts 100 are closed.
The positive output terminal of a vehicle battery 50 may be
connected to the constant current source 52 as was previously
described and through a normally open set of relay contacts 104 to
a two-block indicating circuit generally indicated at 106. The
two-block indicating circuit may include a visual indicator 108, a
relay coil 110 and an audible alarm 112, all of which are arranged
to be connected across the vehicle battery when the relay contacts
104 are closed.
In operation, the meter 96 indicates boom length in response to
changes in the length of the wire 14 as was previously described in
connection with FIG. 4. When the two-block switch 20 is opened, the
relay 76 is energized and the contacts 100 and 104 are closed
thereby bypassing the meter 96 and energizing the two-block circuit
106. The visual indicator 108, the relay coil 110 and the audible
alarm 112 are thus energized providing visual and audible
indications of an impending two-block condition and deenergizing
the hoist cable winch circuitry as was previously described.
Since an electrical signal related to boom length is readily
available from the boom length and anti-two-block circuit 54, this
signal may be utilized, in conjunction with a boom angle related
signal produced by a boom angle indicator and control circuit 118,
to provide load limit calculations as is illustrated in FIG. 6.
Referring now to FIG. 6, the boom of the crane may be mechanically
coupled to the arm of a potentiometer 120 in the boom angle
indicator and control circuit 118. The potentiometer 120 may be
connected between a reference voltage V.sub.REF and ground. The arm
of the potentiometer 120 may be connected through a current
limiting resistor 122 and a meter 124 to the arm of a potentiometer
126.
The potentiometer 126 may be connected as part of a voltage
dividing network comprising resistors 128 and 130 connected in
series with the potentiometer 126 between the reference voltage
V.sub.REF and ground. The meter 124 may include two normally open
switches 132 and 134 which sense and are closed in response to the
respective high and low limits of boom angle. In this regard, the
meter 124 may be any suitable conventional meter having movement
actuated high and low limit switches (e.g. a Simpson model 29XA,
0-50 microamp meter).
One side of each of the switches 132 and 134 may be connected to
the reference voltage V.sub.REF and the other side of each of the
switches may be connected together and through both an audible
alarm 136 and a visual indicator 138 to ground. An output signal
from the arm of the potentiometer 120 may be coupled to a
conventional load limit calculator 140, and the boom length related
signal from the boom length and anti-two-block circuit 54 may also
be supplied to the load limit calculator 140.
In operation, the boom length and anti-two-block circuit 54 senses
a resistance value for the length of the cable 14 between the
contactor assembly 11 and the boom peak 16 as was previously
described. This resistance value is supplied as a length related
electrical signal to the load limit calculator 140 together with a
boom angle related signal from the potentiometer 120. The load
limit calculator employs the boom length signal and the boom angle
signal in a conventional manner to calculate a maximum safe load
limit for a particular sensed boom length and boom angle.
In addition, the meter 124 provides a visual indication of boom
angle. If boom angle reaches either a predetermined high or low
limit, the associated one of the switches 132 and 134 is closed,
applying the reference voltage V.sub.REF to the audible alarm 136
and the visual angle limit indicator 138 to provide the operator
with an indication that a boom angle limit has been reached.
The present invention may be embodied in other specific forms
without departing from the spirit or essential characteristics
thereof. The presently disclosed embodiments are therefore to be
considered in all respects as illustrative and not restrictive, the
scope of the invention being indicated by the appended claims
rather than by the foregoing description, and all changes which
come within the meaning and range of equivalency of the claims are
therefore intended to be embraced therein.
* * * * *